Hard disk drives store information on magnetic disks. Typically, the information is stored on concentric tracks of the disk that are divided into servo sectors and data sectors. Information is written to or read from the disk by a transducer or head, mounted on an actuator arm that positions the transducer over the disk in a predetermined location. Accordingly, the movement of the actuator arm allows the transducer to access the different tracks of the disk. The disk is rotated by a spindle motor at a high speed, allowing the transducer to access different sectors within each track on the disk.
A voice coil motor (VCM) in combination with a servo control system is usually employed to position the actuator arm. The servo control system generally performs the function of seek control and track following. The seek function is initiated when a command is issued to read data from or write data to a target track on the disk. Once the transducer has been positioned sufficiently close to the target track by the seek function of the servo control system, the track following function of the control system centers and maintains the transducer on the target track until the desired data transfer is completed.
Typically, the transducer will oscillate about the center line of the target track for a period of time following the transition of the servo control system from the seek mode to the track following mode. These off-track displacements, or post-seek oscillations (PSO), are due, at least in part, to mechanical vibrations generated by the components of the disk drive during the seek and/or tracking operation. In addition, while in the track following mode, adjustments to the position of the transducer with respect to the center line of the target track are often required due to these same mechanical vibrations. Such adjustments are required to correct drift in the position of the transducer relative to the target track. The precise control of the position of the transducer relative to a target track has become increasingly important as track densities (or tracks per radial inch —TPI) in disk drives have increased. More specifically, the number of tracks included on a disk, i.e., the greater the TPI, translates to higher data storage capability. However, the increased number of tracks means that there is a more stringent requirement that the transducer stay on track for both reading and writing purposes as the separation distance between adjacent tracks decreases. A measure of how far the transducer is off target is termed “Track Misregistration” (TMR). It can be measured in distance (e.g., microns) or as a percentage of track pitch. TMR is also referred to as off track or track following errors.
The actuator assembly also includes a flex circuit that extends from a flex circuit connector mounted to the base plate and electrically interconnected with the disk drive printed circuit board, across a length of the base plate to the actuator, along the actuator arm and suspension and to the transducer for the transfer of information between the transducer and processors located on the printed circuit board. The flex circuit comprises a plurality of conductors or traces embedded in a flexible polyamide material, such as Kapton, that allows the flex circuit to deflect to accommodate the rotary movement of the actuator.
The actuator assembly generally includes one or more actuator arms and a corresponding load beam and slider for each actuator arm, along with the previously described transducer and a single flex circuit which generates vibrational loads that impair the ability of the actuator assembly to position and maintain the transducer over a desired track. The actuator assembly also includes a yoke and voice coil that can also contribute to the vibrational loads. To account for vibrational loads, during the design phase, the amount of vibration from the assembled components may be assigned a budget that must not exceed a predetermined level of generated vibration, thus minimizing TMR and post seek oscillation errors. These budgets are based upon vibrations originating from a number of sources and take on various forms including, but not limited to, electrical noise torque, whirl, arm mode, drum mode, ball bearing tones, high frequency turbulence, disk vibration, aerodynamic torque, and external vibration or seek settle. More specifically, the vibrational loads are generated by the different modes of vibrational motion generated by the components of the actuator assembly. Minute vibrational loads that emanate from the aerodynamic loading of the disk and/or actuator moving through the air inside the disk drive housing may also affect TMR and post seek oscillations. Thus, it is important for designers of disk drives to reduce the individual sources of vibrational loads that influence positioning of the transducer to produce a disk drive with lower vibrational loading such that the servo control system may better compensate for post seek oscillations and TMR.
In addition to the post seek oscillations generated by the components of the actuator assembly, post seek oscillations are also caused by the acceleration and deceleration of the actuator assembly as the actuator arm(s) moves from one track to its intended track. The flex circuit in effect places a torque loading on the actuator assembly. As the actuator moves, resonances in the flex circuit are exited. The primary or first resonant mode is the most distinctive in that it causes the greatest torque disturbance. This is because it is a low frequency resonance, on the order of 200-400 Hz. The magnitude or amplitude of the vibration caused by the flex circuit torque loading is also a problem in PSO. Other sources of post seek oscillation will be apparent to one skilled in the art, such as those from the interaction between various components, such as the bearing and the actuator when the actuator slows or stops the flex circuit and the actuator, rotation of the disk, or the interaction of the voice coil motor with the driver.
The negative effects of post seek oscillations and TMR are most easily described by a brief discussion of track pitch. The distance between two concentric tracks of a disk is known as track pitch, which decreases as TPI increases. For example, a disk with 100,000 TPI has generally a track pitch budget of 0.25 microns (approximately 10 millionths of an inch), wherein a disk with a 150,000 TPI has a track pitch of about 0.17 microns (approximately 7 millionths of an inch). As described above, each vibration-generating component of a disk drive has a budget that contributes to the maximum allowable TMR that are correctable by the servo control system. That is, vibrational induced oscillations of the transducer must be maintained at or below a level where the servo controller can effectively counteract the movement and control the position of the transducer. This level is predetermined in the design of a disk drive. Returning now to the above example in which TPI is increased from 100,000 to 150,000, and the same servo controller is used in each instance, vibrations generated by the flex circuit increase as a percentage of the total budget. Therefore, it is desirable to implement other means of reducing vibrations due to the flex circuit other than through the servo controller.